Python userdata identity can get mixed up in Lua 5.5

[stelonix]

I think I've run into a Python userdata/reference issue with the new lua 5.5 backend.

This reproduces for me with lupa 2.8:

import lupa.lua54 as lua54
import lupa.lua55 as lua55


class Entity:
    def __init__(self, name):
        self.name = name

    def __repr__(self):
        return f"Entity({self.name})"


def test(lua):
    runtime = lua.LuaRuntime()
    echo = runtime.eval("function(entity, event) return entity end")

    entities = [Entity("A"), Entity("B"), Entity("C")]

    for i in range(10000):
        expected = entities[i % 3]
        actual = echo(expected, runtime.table(name="update"))

        if actual is not expected:
            print("failed at", i)
            print("expected:", expected, id(expected))
            print("actual:  ", actual, id(actual))
            return

    print("passed")


print("lua54:")
test(lua54)

print("lua55:")
test(lua55)

For me:

lua54:
passed

lua55:
failed at 51
expected: Entity(A)
actual:   Entity(C)

The exact failing call can move around in a larger repro, but lua55 consistently ends up substituting another live Python object for the one that was passed in. lua54 survives 10,000 calls.

I originally found this in a Lua -> Python -> Lua callback path where an entity argument eventually became another Python object associated with that entity (a dict). I reduced it down to the above, which doesn't need the callback/re-entry at all.

Possibly related to #293, although that one reproduces on 5.2/5.3 and not 5.4+, while this one seems specific to 5.5 for me.

Environment:

Lupa 2.8
Python 3.10.12
Ubuntu 22.04 x86_64
Linux 6.8.0-136-generic #136~22.04.1-Ubuntu SMP PREEMPT_DYNAMIC Fri Jul 3 16:29:11 UTC 2026

I also checked the installed extensions: lupa.lua54 and lupa.lua55 are separate binaries and neither dynamically links a system liblua, so this doesn't look like a 5.4/5.5 shared-library ABI mixup.

For now I've fixed it by using the 5.4 backend.